Stented, radially expandable, tubular PTFE grafts

ABSTRACT

Stented tubular grafts of expanded, sintered polytetrafluoroethylene (PTFE). The stented PTFE grafts of the present invention include an integrally stented embodiment, an externally stented embodiment, and an internally stented embodiment. In each embodiment, the stent may be either self-expanding or pressure-expandable. Also, in each embodiment, the stent may be coated or covered with a plastic material capable of being affixed (e.g., heat fused) to PTFE. Manufacturing methods are also disclosed by the individual components of the stented grafts are preassembled on a mandrel and are subsequently heated to facilitate attachment of the PTFE layer(s) to one another and/or to the stent. Optionally, the stented graft may be post-flexed and post-expanded following it&#39;s removal from the mandrel to ensure that the stented graft will be freely radially expandable and/or radially contractible over it&#39;s full intended range of diameters.

This is a divisional application of U.S. Pat. No. 5,928,279, Ser. No.08/675,644, filed Jul. 3, 1996.

FIELD OF THE INVENTION

The present invention pertains generally to medical devices and theirmethods of manufacture, and more particularly to tubular,polytetrafluoroethylene (PTFE) grafts having integral, radiallyexpandable stents, for implantation in cavities or passageways (e.g.,ducts or blood vessels) of the body.

BACKGROUND OF THE INVENTION

A. Stents

The prior art has included a number of radially expandable stents whichmay be initially deployed in a radially collapsed state suitable fortransluminal insertion via a delivery catheter, and subsequentlytransitioned to a radially expanded state whereby the stent will contactand engage the surrounding wall or the anatomical duct or body cavitywithin which the stent has been positioned. Such stents have been usedto support and maintain the patency of blood vessel lumens (e.g., as anadjuvant to balloon angioplasty) and to structurally support and/oranchor other apparatus, such as tubular endovascular grafts, at desiredlocations within a body cavity or passageway (e.g., to anchor a tubularendovascular graft within a blood vessel such that the graft forms aninternal conduit through an aneurysm or site of traumatic injury to theblood vessel wall).

Many stents of the prior art have been formed of individual member(s)such as a wire, plastic, metal strips, or mesh which has been bent,woven, interlaced or otherwise fabricated into a generally cylindricalconfiguration. These stents of the prior art have generally beenclassified into two major categories—a) “self-expanding” stents, and b)“pressure expandable” stents.

i) Self-expanding Stents

Self-expanding stents are typically formed of spring metal, shape memoryalloy, or other material which is resiliently biased toward its' fullyradially expanded Configuration or otherwise capable of self-expandingto its' fully radially expanded configuration without the need for theexertion of outwardly directed radial force upon the stent by someextraneous expansion apparatus (e.g., a balloon or mechanical expandertool). These self-expanding stents may be initially radially compressedand loaded into a small diameter delivery catheter or alternativelymounted upon the outer surface of a delivery catheter equipped with somemeans for restraining or maintaining the stent in its' radiallycompressed state. Thereafter, the delivery catheter is inserted into thebody and is advanced to a position where the stent is located at or nearthe site at which it is to be implanted. Thereafter, the stent isexpelled out of (or released from) the delivery catheter and allowed toself-expand to it's full radial diameter. Such expansion of the stentcauses the stent to frictionally engage the surrounding wall of the bodycavity or passageway within which the stent has been positioned. Thedelivery catheter is then extracted, leaving the self-expanded stent atits' intended site of implantation. Some examples of self-expandingstents of the prior art include those described in U.S. Pat. No.4,655,771 (Wallsten et al.); U.S. Pat. No. 4,954,126 (Wallsten): U.S.Pat. No. 5, 061, 275 (Wallsten et al.); U.S. Pat. No. 4,580,568(Gianturco); U.S. Pat. No. 4,830,003 (Wolf et al.); U.S. Pat. No.5,035,706 (Gianturco et al.) and U.S. Pat. No. 5,330,400 (Song).

ii) Pressure-Expandable Stents

The pressure-expandable stents of the prior art are typically formed ofmetal wire, metal strips, or other malleable or plastically deformablematerial, fabricated into a generally cylindrical configuration. Thepressure-expandable stent is initially disposed in a collapsedconfiguration having a diameter which is smaller than the desired finaldiameter of the stent, when implanted in the blood vessel. The collapsedstent is then loaded into or mounted upon a small diameter deliverycatheter. The delivery catheter is then advanced to its desired locationwithin the vasculature, and a balloon or other stent-expansion apparatus(which may be formed integrally of or incorporated into the deliverycatheter) is utilized to exert outward radial force on the stent,thereby radially expanding and plastically deforming the stent to it'sintended operative diameter whereby the stent frictionally engages thesurrounding blood vessel wall. The material of the stent undergoesplastic deformation during the pressure-expansion process. Such plasticdeformation of the stent material causes the stent to remain in itsradially expanded operative configuration. The balloon or otherexpansion apparatus is then deflated/collapsed and is withdrawn from thebody separately from, or as part of, the delivery catheter, leaving thepressure-expanded stent at it's intended site of implantation.

Some examples of pressure-expandable stents of the prior art includethose described in U.S. Pat. No. 5,135,536 (Hillstead); U.S. Pat. No.5,161,547 (Tower); U.S. Pat. No. 5,292,331 (Boneau); U.S. Pat. No.5,304,200 (Spaulding) and U.S. Pat. No. 4,733,665 (Palmaz).

B. PTFE Vascular Grafts

Fluoropolymers, such as polytetrafluoroethylene, have been heretoforeused for the manufacture of various types of prosthetic vascular grafts.These vascular grafts are typically of tubular configuration so as to beuseable to replace an excised segment of blood vessel.

The tubular PTFE vascular grafts of the prior art have traditionallybeen implanted, by open surgical techniques, whereby a diseased ordamaged segment of blood vessel is surgically excised and removed, andthe tubular bioprosthetic graft is then anastomosed into the host bloodvessel as a replacement for the previously removed segment thereof.Alternatively, such tubular prosthetic vascular grafts have also beenused as bypass grafts wherein opposite ends of the graft are sutured toa host blood vessel so as to form a bypass conduit around a diseased,injured or occluded segment of the host vessel.

In general, many tubular prosthetic vascular grafts of the prior arthave been formed of extruded, porous PTFE tubes. In some of the tubulargrafts of the prior art a PTFE tape is wrapped about and laminated tothe outer surface of a tubular base graft to provide reinforcement andadditional burst strength. Also, some of the prior tubular prostheticvascular grafts have included external support member(s), such as a PTFEbeading, bonded or laminated to the outer surface of the tubular graftto prevent the graft from becoming compressed or kinked duringimplantation. These externally supported tubular vascular grafts haveproven to be particularly useful for replacing segments of blood vesselwhich pass through, or over, joints or other regions of the body whichundergo frequent articulation or movement.

One commercially available, externally-supported, tubular vascular graftis formed of a PTFE tube having a PTFE filament helically wrappedaround, and bonded to, the outer surface of the PTFE tube. (IMPRA Flex™Graft, IMPRA, Inc., Tempe, Ariz.)

One other commercially available, externally-supported, tubular vasculargraft comprises a regular walled, PTFE tube which has PTFE reinforcementtape helically wrapped around, and bonded to,the outer surface of thePTFE tube and individual rings of Fluorinated Ethylene Propylene (FEP)rings disposed around, and bonded to, the outer surface of thereinforcement tape. (FEP ringed ePTFE vascular graft, W. L. Gore &Associates, Inc., Flagstaff, Ariz.)

C. Stented Grafts

The prior art has also included a number of “stented grafts”. Thesestented grafts typically comprise a self-expanding orpressure-expandable stent which is affixed to or formed within a pliabletubular graft. Because of their radial compressibility/expandability,these stented grafts are particularly useable in applications wherein itis desired to insert the graft into an anatomical passageway (e.g.,blood vessel) while the graft is in a radially compact state, and tosubsequently expand and anchor the graft to the surrounding wall of theanatomical passageway.

More recently, methods have been developed for introducing andimplanting tubular prosthetic vascular grafts within the lumen of ablood vessel, by percutaneous or minimal incision means. Suchendovascular implantation initially involves translumenal delivery ofthe graft, in a compacted state, by way of a catheter or othertransluminally advancable delivery apparatus. Thereafter, the graft isradially expanded and anchored to the surrounding blood vessel wall,thereby holding the graft at its intended site of implantation withinthe host blood vessel. An affixation apparatus such as a stent, istypically utilized to anchor at least the opposite ends of the tubulargraft to the surrounding blood vessel wall. One particular applicationfor endovascular grafts of this type is in the treatment of vascularaneurysms without requiring open surgical access and resection of theaneurysmic blood vessel. Also, such stented grafts may also be useableto treat occlusive vascular disease—especially in cases where thestented graft is constructed in such a manner that the tubular graftmaterial forms a complete barrier between the stent and the blood whichis flowing through the blood vessel. In this manner the tubular graftmaterial may serve as a smooth, biologically compatible, inner“covering” for the stent, thereby preventing a) turbulent blood-flow asthe blood flows over the wire members or other structural material ofwhich the stent is formed, b) immunologic reaction to the metal or othermaterial of which the stent is formed, and c) a barrier to separate adiseased or damaged segment of blood vessel from the blood-flow passingtherethrough. Such prevention of turbulent blood-flow and/or immunologicreaction to the stent material is believed to be desirable as both ofthese phenomena are believed to be associated with thrombus formationand/or restenosis of the blood vessel.

Other uses for stented grafts may include restoring patency to, orre-canalizing, other anatomical passageways such as ducts of the biliarytract, digestive tract and/or genito-urinary tract.

Many of the stented grafts known in the prior art have utilized woven orknitted material, such as polyester fiber, as the graft material.

There exists a need for the development of a radially expandable,stented graft formed of a continuous, tubular ePTFE tube because theinherent properties of PTFE may offer various clinical advantages overthe woven polyester and other graft materials which have been previouslyused in stented grafts of the prior art.

SUMMARY OF THE INVENTION

The present invention is directed to stented, tubular, PTFE grafts andtheir methods of manufacture. In general, the present invention mayexist in any of three (3) separate embodiments, depending upon whetherthe stent component of the graft is formed integrally (i.e., within) thetubular PTFE graft, externally (i.e., on the outer surface of) thetubular PTFE graft, or internally (i.e., on the inner lumenal surface)of the PTFE tubular graft. Each of these three separate embodiments ofthe invention may be self expanding (i.e., incorporating aself-expanding stent) or pressure-expandable (i.e., incorporating apressure-expandable stent.

In accordance with a first embodiment of the invention, there isprovided an integrally stented PTFE graft which comprises a tubular PTFEbase graft preferably of a density less than 1.6 g/cc, a radiallyexpandable stent surrounding the outer surface of the tubular basegraft, and an outer PTFE layer having a density of less than 1.6 g/cc.The tubular outer layer is fused to the tubular base graft throughlateral openings or perforations formed in the stent. A polymer coating,such as a PTFE coating, may be disposed on the stent to furtherfacilitate fusing or boding of the stent to the base tube and/or outertubular layer.

In accordance with a second embodiment of the invention, there isprovided an externally stented, tubular PTFE graft which comprises aradially compressible/expandable stent having a ePTFE tube of less than1.6 g/cc density coaxially disposed within the stent, with the outersurface of the tubular ePTFE graft being fused or attached to the stent.A polymer coating, such as PTFE or any other plastic which may be fusedor adhered to PTFE, may be applied to or formed on the stent tofacilitate the desired fusion or attachment of the tube graft to thestent, and/or to improve the biocompatibility of the stent.

In accordance with a third embodiment of the invention, there isprovided an internally stented, tubular PTFE graft comprising a tubularouter layer formed of ePTFE having a density of less than 1.6 g/cc, anda radially expandable stent. The stent is coaxially disposed within thelumen of the tubular outer layer, and fused or attached thereto. Thestent may be covered with a polymer coating, such as PTFE or otherbiocompatable plastic capable of adhering or fusing to PTFE, tofacilitate the desired fusion or attachment of the stent to the outertubular layer, and/or to improve the biocompatability of the stent.Additionally or alternatively, PTFE particles may be disposed betweenthe tubular outer layer and the tubular base graft to facilitateadhering or fusing of these two layers to one another, and/or to thestent. Such PTFE particles may be disposed between the inner base graftand outer tubular layer by applying or depositing PTFE liquid dispersiontherebetween, or by depositing dry PTFE resin powder therebetween.

Any of above-summarized three (3) separate embodiments of the inventionmay be manufactured by a method which comprises the steps of: a)initially positioning a generally cylindrical stent of either theself-expanding or pressure-expandable variety in contacting coaxialrelation with the tubular ePTFE base graft and/or the tubular ePTFEouter layer, upon a cylindrical mandrel or other suitable supportsurface, and b) subsequently fusing the fuse (i.e., heating to alamination temperature) assembled components (i.e., the stent incombination with the inner base graft and/or outer tubular layer) of thestented graft into a unitary stented graft structure. In integrallystented embodiments where both the tubular ePTFE base graft and thetubular ePTFE outer layer are present, such heating will. additionallycause the tubular outer layer to fuse to the inner tubular base graft,through lateral openings or perforations which exist in the stent. Thestent may be surface treated, abraded, or coated with a plastic capableof adhering or fusing to ePTFE to facilitate attachment of the stent tothe adjacent outer layer and/or inner base graft upon subsequentheating, application of solvent or other suitable adhesion promotingtechnique. In instances where a plastic coating is formed on the stent,such coating may be in the nature of a tube or film which is applied tothe stent prior to assembly and mounting of the stented graft componentson the mandrel or other support surface. Also, in embodiments where boththe outer tubular layer and tubular base graft are used, aqueous PTFEdispersion, powdered PTFE resin or other flowable plastic material, maybe deposited between the outer tubular layer and inner tubular basegraft at the time of assembly (prior to heating) to further facilitatefusion of the outer tubular layer and/or inner tubular base graft to thestent and/or to one another.

By the above-described materials and methods of construction, thestented PTFE grafts of the present invention are capable of radiallyexpanding and contracting without excessive puckering, wrinkling orinvagination of the PTFE graft material. Furthermore, in embodimentswherein the stent is constructed of individual members which move orreposition relative to one another during respective expansion andcontraction of the stented graft, the manufacturing methods andmaterials of the present invention render the PTFE sufficiently strongand sufficiently firmly laminated or fused so as to permit such relativemovement of the individual members of the stent without tearing orrupturing of the tubular PTFE graft.

Further objects and advantages of the invention will become apparent tothose skilled in the art upon reading and understanding the followingdetailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an integrally stented PTFE tubular graftof the present invention, wherein a portion of the graft has beeninserted into a tubular catheter.

FIG. 1a is an enlarged perspective view of a segment of FIG. 1.

FIG. 2 is an enlarged, cut-away, elevational view of a preferred,integrally stented, tubular PTFE graft of the present invention.

FIG. 3a is an enlarged perspective view of a portion of the stentincorporated in the graft of FIG. 2.

FIG. 3b is an enlarged cross-sectional view through line 3 b-3 d of FIG.3a.

FIGS. 4a-4 f are a step-by-step illustration of a preferred method formanufacturing an integrally stented PTFE graft of the present invention.

FIG. 5 is a schematic illustration of an alternative electron beamdeposition method which is usable for depositing PTFE coating on thestent portion of the integrally stented PTFE grafts of the presentinvention.

FIG. 6 is a perspective view of an alternative heating apparatus whichis useable in the manufacture of the integrally stented PTFE grafts ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description is provided for the purpose ofdescribing and illustrating presently preferred embodiments of theinvention only, and is not intended to exhaustively describe allpossible embodiments in which the invention may be practiced.

A. The Structure of an Integrally Stented PTFE Graft

With reference to FIGS. 1-3b, there is shown an integrally stentedtubular PTFE graft 10 of the present invention. The preferred integrallystented graft 10 comprises a tubular PTFE base graft 12, a PTFE-coatedstent 14 and an outer layer of PTFE 16.

One of the many types of stents which may be used to form the stent 14component of a stented graft 10 of the present invention, is shown inthe drawings. This particular stent 14 is formed of individual elementsor wires 18 which have been coated with a PTFE coating 20. Gaps orlateral openings 19 exist between adjacent ones or bundles of the wires18. The configuration, construction, and function of this stent 14 isdescribed in detail in U.S. Pat. No. 4,655,771 Wallsten); U.S. Pat. No.4,954,126 (Wallsten); and U.S. Pat. No. 5,061,275 (Wallsten et al.), theentireties of which are hereby expressly incorporated herein byreference. As shown in the figures of this patent application, thisparticular stent 14 is composed of rigid but resiliently flexible threadelements or wires 18. These thread elements or wires 18 are formed ofmetal, such as an alloy of cobalt, chromium, nickel or molybdenum,wherein the alloying residue is iron. One specific example of acommercially available alloy which may is usable to form the wires 18 ofthe stent 14 is Elgiloy (The Elgiloy Company, 1565 Fleetwood Drive,Elgin, Ill. 60120). The wires 18 of this stent 14 are arranged inhelical configuration about a common longitudinal axis LA. A number ofthe wires 18 are positioned in substantially parallel relation to oneanother, but are axially displaced relative to each other. By sucharrangement, some of the wires 18 are wound in a first helicaldirection, while others are wound in a second or opposite helicaldirection such that they cross on opposite sides of adjacent ones of thewires wound in the first helical direction so as to form a helicallybraided wire stent as shown in the Figures. This results in theformation of a braided wire stent 14 of generally tubular configurationwhich is self-expanding and biased to it's radially expanded diameterD₂. However, this stent 14 may be radially compressed to a smallerdiameter D₁ and radial constraint, as may be applied by the surroundingwall of the tubular delivery catheter 22 shown in FIG. 1, may be appliedto hold the stent 14 in such radially compressed state (diameter D₁).Thereafter, when the radial constraint is removed from the stent 14, thestent 14 will resiliently spring back to its radially expanded diameterD₂. The individual, helically wound wires 18 of this particular braidedstent 14 move and articulate such that the angular dispositions of thewires 18, relative to one another, will change radial expansion andcompression of the stent 14. Also, the longitudinal length of the stent14 will increase as the stent 14 is radially compressed toward itsradially compact configuration D₁, and such length will shorten as thestent 14 expands toward its radially expanded configuration D₂. Thus, isthe optional PTFE coating 20 is applied to the wires 18 of the stent 14,such coating (described in detail herebelow) is preferably flexibleenough to withstand the flexing and movement of the individual wires 18without cracking or degrading.

The tubular base graft 12 is initially coaxially positioned within thehollow inner bore of the tubular stent 14 while the stent 14 is in it'sradially expanded configuration, after the stent 14 has been coated withthe PTFE coating 20, if desired. Thereafter, the outer PTFE layer 16 isformed by any suitable means, such as by wrapping PTFE tape 17 upon theouter surface of the PTFE coated stent 14 to form the generally tubularouter PTFE layer 16. Thereafter, heat or other means are utilized tofuse the outer PTFE layer 16 to the inner base graft 12, through thegaps or openings 19 which exist in the stent 14. In embodiments whereinthe optional PTFE coating 20 has been applied to the stent 14, suchheating will also facilitate bonding of the PTFE coating 20 of the stent14 to the adjacent base graft 12 and outer PTFE layer 16. In thismanner, there is formed a self-expanding, tubular, integrally stented,PTFE graft 10 of substantially unitary construction. The stent 14 formsan integral structural framework within the tubular graft 10, and thefused PTFE body of the graft is low enough in density and sufficientlypliable to allow the stent 14 incorporated into the graft 10 to continueto undergo substantially the same range of radial expansion andcontraction that such stent 14 was capable of before disposition, of thePTFE graft thereon. In this regard, the internally stented graft 10 isradially compressible to the stent's first diameter D₁ and may be may beinserted into the lumen of a small diameter tubular catheter 22. Theexternal constraint provided by the wall of the catheter 22 willmaintain the stented graft 10 in it's radially compressed configurationof diameter D₁ until such time as the graft 10 is expelled or ejectedout of the catheter 22. After the graft 10 has been expelled or ejectedout of the catheter 22, the graft will self-expand to a diameter whichis substantially equal to the original expanded diameter D₂ of the stent14.

B. Preparation of the Tubular Base Graft

i.) Preparation of Paste

The manufacture of the tubular base graft begins with the step ofpreparing a PTFE paste dispersion for subsequent extrusion. This PTFEpaste dispersion may be prepared by known methodology whereby a fine,virgin PTFE powder (e.g., F-104 or F-103 Virgin PTFE Fine Powder, DakinAmerica, 20 Olympic Drive, Orangebury, N.Y. 10962) is blended with aliquid lubricant, such as odorless mineral spirits (e.g., Isopar®, ExxonChemical Company, Houston, Tex. 77253-3272), to form a PTFE paste of thedesired consistency.

ii.) Extrusion of Tube

The PTFE-lubricant blend dispersion is subsequently passed through atubular extrusion dye to form a tubular extrudate.

iii.) Drying

The wet tubular extrudate is then subjected to a drying step whereby theliquid lubricant is removed. This drying step may be accomplished atroom temperature or by placing the wet tubular extrudate in an ovenmaintained at an elevated temperature at or near the lubricant's drypoint for a sufficient period of time to result in evaporation ofsubstantially all of the liquid lubricant.

iv.) Expansion

Thereafter, the dried tubular extrudate is longitudinally expanded orlongitudinally drawn at a temperature less than 327° C. and typically inthe range of 250-326° C. This longitudinal expansion of the extrudatemay be accomplished through the use of known methodology, and may beimplemented by the use of a device known as a batch expander. Typically,the tubular extrudate is longitudinally expanded by an expansion ratioof more than two to one (2:1) (i.e., at least two (2) times its originallength).

Preferably, the base graft 12 is formed of expanded, sintered PTFEhaving a density of less than 1.6 grams per cubic centimeter.

v.) Sintering

After the longitudinal expansion step has been completed, the expandedPTFE tube is subjected to a sintering step whereby it is heated to atemperature above the sintering temperature of PTFE (i.e., 350-370° C.)to effect amorphous-locking of the PTFE polymer. The methodology used toeffect the sintering step, and the devices used to implement suchmethodology, are known in the art.

Completion of the sintering step marks the completion of the preparationof the expanded, sintered PTFE base graft 12.

The PTFE tape 16 may be manufactured by any suitable method, includingthe general method for manufacturing expanded PTFE tape, as follows:

C. Preparation of PTFE Tape

i.) Preparation of Paste Dispersion

The usual manufacture of an expanded, sintered PTFE tape 17 useable toform the outer PTFE layer 16 of the stented graft 10 begins with thepreparation of a PTFE paste dispersion. This PTFE paste dispersion maybe prepared in the same manner as described hereabove for preparation ofthe PTFE paste dispersion used to form the tubular base graft.

ii.) Extrusion of Film

The PTFE paste dispersion is subsequently passed through the filmextrusion dye to form a wet film extrudate. The wet film extrudate istaken up or wound upon a rotating core so as to form a roll of the wetfilm extrudate.

iii.) Calendaring

The wet film extrudate is subsequently unrolled and subjected to aninitial cold (i.e., <100° C.) calendaring step by passing the filmthrough at least one set of opposing stainless steel calendaring rollershaving an adjustable gap thickness therebetween. The calendaring rollersare preferably maintained at a temperature between room temperature and60° C. The width of the wet extrudate is held constant as it passesthrough these calendaring rollers. The thickness of the wet filmextrudate is reduced to its desired final thickness (e.g., 0.004-0.005inches) while the width of the film is maintained constant. It will beappreciated that, since the width of the film is maintained constant,the passage of the film through the calendaring machine will result in alongitudinal lengthening of the film. The amount of longitudinallengthening will be a function of the decrease in film thickness whichoccurs as the film passes between the calendaring rollers.

One example of a commercially available calendaring machine useable forthis purpose is the small Killion 2 Roll Stack, (Killion Extruders,Inc., Cedar Grove, N.J. 07009.)

iv) Drying

Thereafter, the wet film is subjected to a drying step. This drying stepmay be accomplished by permitting or causing the liquid lubricant toevaporate from the matrix of the film. Such evaporation of the liquidlubricant may be facilitated by passing the film over a drum or rollerwhich is maintained in an elevated temperature sufficient to cause theliquid lubricant to fully evaporate from the film matrix.

v) Expansion

Separately, or concurrently with the drying step the film is subjectedto an expansion step. Such expansion step comprises expanding the PTFEfilm in at least one direction (e.g., longitudinally). Such expansion ofthe film serves to a) increase the porosity of the film, b) increase thestrength of the film, and c) orient the PTFE fibrils in the direction ofthe axis of expansion.

This expansion step is typically carried out with some heating of thefilm during such expansion, but such heating does not exceed thecrystalline melting point of the PTFE polymer.

vi) Sintering of the Film

After the drying step and expansion step have been completed, the filmis subjected to a sintering step wherein the film is heated to atemperature above the melting point of PTFE to accomplish sintering oramorphous locking of the PTFE polymer. This sintering step may becarried out by passing the film over a drum or roller which ismaintained at a high surface temperature (e.g., 350—420° C.) to causethe desired heating of the PTFE film above the melting point of the PTFEpolymer for a sufficient period of time to effect the desired sinteringof the film.

vii) Cutting the Film Into Strips

After the film has been dried, the film is cut into strips, each striptypically having a width of 0.25-0.50 inches, thereby creating strips ofexpanded, sintered PTFE tape 14.

D. Coating of the Stent and/or Deposition of PTFE Between Layers toEnhance Bonding

Prior to assembly of the components of the integrally stented graft 10,the stent 14 may be coated with a polymer coating 20.

The polymer coating formed on the stent 14 may be any suitable type ofpolymer which will adhere to PTFE. Examples of polymers which may beused for such polymer coating or covering includepolytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),polytetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA),polyvinyl chloride (PVC), polypropylene (PP), polyethylene terephthalate(PET) polyvinylidene fluoride (PVDF) and other biocompatable plastics.

One manner in which such coating of the stent 14 may be carried out isillustrated in FIG. 4a. As shown in FIG. 4a, the stent 14 may beimmersed in a vessel 30 containing an aqueous dispersion of PTFE 32. Oneaqueous PTFE dispersion which may be useable for coating of the stent 14is Dupont T-30 Aqueous PTFE dispersion, available commercially from theE.I. DuPont de Nemours Co., (Wilmington, Delaware). Another commerciallyavailable PTFE dispersion 32 which may be utilized for coating of thestent is Daikin-Polyflon TFE Dispersion, available PTFE dispersion 32which may be utilized for coating of the stent is Dakin-polyflon TFEDispersion, available from Daikin Industries, Ltd., Chemical Division(Umeda Center Bldg., 4-12 chome, Nakazaki-nishi, Kita-ku, Osaka, Japan).

The time in which the stent 14 must remain immersed in the liquid PTFEdispersion 32 may vary, depending on the construction of the stent 14and the chemical composition of the PTFE dispersion 32. However, in mostcases, an immersion time of 10-15 seconds will be sufficient to obtainuniform deposition of the PTFE coating 20 on the wire members 18 of thestent 14.

After the stent 14 has been removed from the liquid PTFE dispersion 32,it will be permitted to air dry such that a dry PTFE coating 20 remainsdeposited upon the outer surface of each wire 18 of the stent 14.

Optionally, after the air drying has been completed, the PTFE coatedstent 14 may be placed in an oven at 350° C. for approximately 10minutes to sinter the PTFE coating and/or to enhance the bonding of thePTFE coating 20 to wire members 18 of the stent 14. Sintering of thePTFE coating renders the coating more resistant to abrasion or peelingduring the subsequent handing of the stent and/or the ensuingmanufacture and use of the stented graft 10. It will be appreciated thatvarious alternative methods, other than immersion, may be used fordepositing the PTFE coating 20 on the stent 14. One alternative methodis electron beam deposition, as illustrated in FIG. 5. In accordancewith this alternative PTFE deposition method, the stent 14 is positionedwithin a closed vacuum chamber 36 wherein a mass of PTFE 38 is located.An electron beam apparatus 40 is then utilized to project electron beamradiation onto the PTFE 38 within the chamber 36 so as to causesublimation of the PTFE and resultant deposition of the layer 20 of PTFEon the outer surface of the stent 14. The apparatus and specificmethodology useable to perform this electron beam deposition of the PTFEcoating 20 are well known to those of skill in the relevant art.

As with the above-described immersion process (FIG. 4a), the stent 14whereupon the PTFE coating 20 has been deposited may be subjected tooptional heating at 350° C. for a period of approximately ten minutes inorder to sinter the PTFE coating and/or to enhance the bonding of thePTFE coating 20 to the wire members 18 of the stent 14.

As an alternative to coating of the stent, or in addition thereto, suchPTFE aqueous dispersion may be painted onto the outer surface of thebase graft 12, or the inner surface of the outer tubular layer 16, ormay be otherwise disposed between the base graft 12 and outer tubularlayer 16 to facilitate fusion or bonding of the inner base graft 12 tothe outer tubular layer 16. Or, such PTFE aqueous dispersion may besprayed or otherwise applied to the outer surface of the outer tubularlayer 16 provided that the PTFE present in the dispersion are smallenough to migrate inwardly through pores in the outer tubular layer 16,thereby becoming deposited between the outer tubular layer 16 and theinner base graft 12.

Another alternative or additional means by which adherence or fusion ofthe base graft 12, outer tubular layer 16 and/or stent 14 may befacilitated or enhanced includes the deposition of raw PTFE resin powderbetween the outer tubular layer 16 and inner base graft 12, and/or uponthe stent 14.

It will be appreciated that in many cases, it will be desirable to applythe polymer coating 20 to the stent 14 while the stent 14 is in it'sfully radially expanded configuration of diameter D₂. In this manner,after the coating 20 has been applied and formed on the fully radiallyexpanded stent 14, the stent 14 may subsequently be contracted to it'sradially compact configuration of diameter D₁ without tearing ordisrupting of the previously-applied coating 20. In embodiments whichutilize a pressure-expandable stent 14, it may thus be necessary tovolitionally or purposely expand the stent 14 to it's fully radiallyexpanded diameter D₂ prior to application of the coating 20.Alternatively, when the stent 14 is of the self-expanding variety itwill, in most cases, automatically assume it's fully radially expandedconfiguration of diameter D₂ and no such volitional or purposefulpre-expansion of the stent 14 will be required.

The preferred means by which liquid PTFE dispersion and/or solid PTFEpowder may be deposited between the outer tubular layer 16, and innerbase graft 12 will be discussed in more detail herebelow with referenceto the manufacturing methodology.

E. Assembly and Construction of the Integrally Stented PTFE Graft

FIGS. 4b-4 f show, in step-wise fashion, the preferred method forassembling and constructing the integrally stented PTFE graft 10.

As shown in FIG. 4b, the tubular base graft 12 is initially disposed ona rod or mandrel 50. Such rod or mandrel 50 may comprise a stainlesssteel rod having an outer diameter which is only slightly smaller thanthe inner diameter of the tubular base graft 12. In this manner, thetubular base graft 12 may be slidably advanced onto the outer surface ofthe mandrel 50 without undue effort or damage to the base graft 12.

Thereafter, the PTFE-coated stent 14 is axially advanced onto the outersurface of the tubular base graft 12, as shown in FIG. 4c.

At this point in the process, PTFE liquid dispersion or powdered PTFEresin may be additionally (optionally) applied to the stent 14 and/orouter surface of the base graft 12 to promote further bonding and fusionof the base graft 12 to the stent 14 and/or subsequently applied outerlayer 16. In this regard, the mandrel-borne tubular base graft 12 andstent 14 may be rolled in powdered PTFE resin to accomplish the desireddeposition of PTFE powder thereon. Alternatively, the above-describedPTFE liquid dispersion may be painted sprayed or otherwise applied tothe surface of the stent 14 and/or outer surface of the tubular basegraft 12 prior to subsequent application of the outer tubular layer 16.

Thereafter, as shown in FIG. 4d, the tape 17 is initially helicallywrapped in overlapping fashion, on the outer surface of the stent 14, ina first direction. In the preferred embodiment, tape of ½ inch width isused. The tape is helically wrapped about the stent at a pitch anglewhereby 6 to 8 revolutions of the tape are applied per linear inch ofthe stent 14.

Thereafter, as shown in FIG. 4e, a second tape wrap in the oppositedirection is accomplished, preferably using the same width of tape atthe same pitch angle, thereby applying another 6-8 revolutions of tape17 per linear inch of stent 14. In this manner, both wrappings of thetape 17 (FIGS. 4d and 4 e) combine to form a tubular, outer PTFE layer16 which preferably has a thickness of less than 0.1 inches, and whichmay be formed of 1 to 10 consecutive (e.g., laminated) layers of thetape 17. for example, when using ePTFE tape of less than 1.6 g/ccdensity and ½ inch width, the first helical wrap (FIG. 4d) may depositfour consecutive layers of tape 17 and the second helical wrap (FIG. 4e)may deposit an additional 4 layers of tape 17, thereby resulting in anouter tubular layer 16 which is made up of a total of 8 layers of suchtape 17.

Optionally, to further promote bonding of the outer tubular layer 16 tothe stent 14 and/or inner base graft 12, liquid PTFE dispersion may besprayed, painted or otherwise applied to and dried upon the tape 17prior to wrapping, or such liquid PTFE dispersion may be deposited byany suitable means (spraying, painting, etc.) between the outer tubularlayer 16 formed by the helically wrapped tape 17 and the inner basegraft 12. Or such liquid PTFE dispersion may be sprayed onto orotherwise applied to the outer surface of the helically wrapped tape 17such the small particles of PTFE contained within the liquid dispersionwill migrate inwardly through pores in the layers of tape 17, and willthereby become deposited between the outer tubular layer 16 and theinner base graft 12 prior to subsequent heating of the assembly, asdescribed herebelow. Another alternative (and optional) method fordepositing polymer (e.g., PTFE) particles between the base graft 12 andouter tubular layer 16 is by rolling the mandrel 50, having the basegraft 12 and stent 14 disposed thereon, in dry, powdered polymer resin(e.g., the above-described PTFE resin) to cause such dry polymer resinto become deposited on the outer surface of the base graft 12 and/orstent 14 prior to application of the tape 17 as shown in FIGS. 4d and 4e.

Thereafter, as shown in FIG. 4f, ligatures 52 of stainless steel wireare tied about the opposite ends of the graft 10 so as to securely holdthe base graft 12, PTFE-coated stent 14 and outer layer 16 on themandrel 50. The mandrel, having the graft 10 disposed thereon is thenheated to a temperature of 363°±2° C. for thirty minutes. Such heatingwill cause the outer PTFE layer 16 to heat fuse to the inner base graft12 through the openings 19 which exist in the stent 14, and will furtherfacilitate bonding or fusing of the PTFE coating 20 of the stent 14 tothe adjacent base graft 12 and outer tape layer 16. In this manner, thedesired integrally-stented PTFE tubular graft 10 is formed.

The heating step illustrated schematically in FIG. 4f may be carried outby any suitable means. For example, the mandrel 50 having the graft 10and ligatures 52 disposed thereon may be placed in an oven preheated tothe desired temperature, for the desired period of time. Alternatively,the mandrel, having the graft 10 and ligatures 52 disposed thereon maybe rolled on a hot plate or heated surface to accomplish the desiredheat fusing or bonding of the outer layer 16, base graft 12 and PTFEcoating 20 of the stent 14.

Another alternative apparatus which may be utilized for the heating stepshown schematically in FIG. 4f, is the U-shaped aluminum block heatershown in FIG. 6. This aluminum block heater is formed of a solidaluminum plate or block 54 formed into a generally U-shapedconfiguration, and having a plurality of bore holes 60 formedlongitudinally therein and extending at least part way therethrough.Elongate, cylindrical, electric heaters 62, such as those commerciallyavailable from the Watlow Electric Company, 12001 Lackland Road, St.Louis, Mo. 63146, are inserted into the bore holes 60, and such heaters62 are heated to a temperature which will cause the inner surface of theU-shaped aluminum heater block 54 to be maintained at approximately 300°C. or greater. It will be appreciated that other types of heatingapparatus, such as electrical strip heaters mounted on the outer surfaceof the U-shaped block 54, may be useable as an alternative to the boreholes 60 and cylindrical heaters 62 described herein.

After the U-shaped block 54 has been heated to the desired temperature,the mandrel 50, having the graft 10 and ligatures 52 disposed thereon,is inserted into the U-shaped inner region of the block 54, and isrotated, therein so as to accomplish the desired heat fusing of thetubular base graft 12, outer tape layer 16 and PTFE coating 20 of thestent 14.

In many applications, it will be desirable to post-flex and re-expandthe stented graft 10 to ensure that the stented graft 10 is capable ofundergoing full radial compression and full radial expansion, over it'scomplete range of intended diameters.

To accomplish this post-flexing and re-expansion of the stented graft10, the stented graft 10 is removed from the mandrel 50 and is held in aheated environment, such as in the inner space of the U-shaped heaterdevice shown in FIG. 6. Thereafter, the opposite ends of the stent 14are pulled longitudinally away from each other to thereby radiallycontract the stented graft 10 to it's minimal radially compresseddiameter D₁. Thereafter, the stented graft 10 is allowed to self-expand.If this self-expansion of the stented graft 10 does not result in returnof the stented graft 10 to its fully radially expanded diameter D₂, thestented graft 10 may then be re-advanced onto the mandrel 50 to therebyforce the stented graft 10 to reassume it's full radially expandedconfiguration of diameter D₂.

Thereafter, when the graft is again removed from the mandrel 50, thestented graft 10 will be rendered capable of being radially compressedto it's fully compressed diameter D₁, and subsequently self-expanded toit's full radially expanded diameter D₂.

F. Assembly and Construction of Internally Stented PTFE Tube Graft

In a first alternative embodiment of the invention, the inner base graft12 may be eliminated or excluded, thereby providing a modified versionof the stented graft 10 comprising only the stent 14 and outer tubularlayer 16.

In this first alternative embodiment, the above-described manufacturingmethod is performed as described without the tubular base graft 12,thereby forming a modified version of the stented graft 10 wherein theouter tubular layer 16 of PTFE is fused only to the stent 14.

In embodiments wherein the stent 14 is coated with a polymer coatingsuch as PTFE, the presence of such coating on the stent 14 will providelubricity and biocompatability, which may render such internally stentedgraft suitable for use in applications wherein the exposed stent 14 willcome in direct contact with biological fluid or blood flowing throughthe graft, thereby avoiding the need for use of the internal base graft12.

Thus, this first alternative embodiment of the present inventionincludes all possible embodiments wherein only the outer tubular layer16 is utilized in conjunction with the stent 14, to provide aninternally stented graft 10 which is devoid of any internal tubular basegraft 12.

G. Assembly and Construction of Externally Stented PTFE Tube Graft

In a second alternative embodiment of the invention, the outer tubularlayer 16 may be excluded or eliminated, thereby providing an externallystented PTFE tube graft which comprises only the stent 14 and theinner-base tube 12.

In this second alternative embodiment, the above-described manufacturingmethod is performed as described without the outer tubular layer 16.This results in the formation of a modified version of the stented graft10, comprising only the inner base graft 12 and the stent 14.

In embodiments wherein the stent 14 is coated with a polymer coating,such as PTFE, the presence of such coating on the stent 14 will providefor enhanced biocompatability, which may render such externally stentedgraft suitable for implantation in blood vessels or other tubularanatomical passageways wherein the exposed exterior of the coated stent14 comes in direct contact with vascular tissue or other tissue of thebody, thereby avoiding the need for use of the outer tubular layer 16.

Thus, this second alternative embodiment of the present inventionincludes all possible embodiments wherein only the inner base graft 12,is utilized in conjunction with the stent 14, to provide an externallystented graft 10 which is devoid of any outer tubular layer 16.

It will be appreciated that the invention has been described hereabovewith reference to certain presently preferred embodiments of theinvention. Various additions, deletions, alterations and modificationsmay be made to the above-described embodiments without departing fromthe intended spirit and scope of the invention. Accordingly, it isintended that all such reasonable additions, deletions, modificationsand alterations to the above described embodiments be included withinthe scope of the following claims.

What is claimed is:
 1. A method for manufacturing an integrally stented,tubular, PTFE graft that is alternately deployable in a radially compactconfiguration having a first diameter and a radially expandedconfiguration having a second diameter, said method comprising the stepsof: a) extruding a PTFE paste to form a tubular PTFE extrudate; b)longitudinally expanding the tubular PTFE extrudate to form a tubularbase graft; c) positioning the tubular base graft on a generallycylindrical mandrel; d) providing a generally cylindrical stent having alongitudinal bore extending therethrough, wherein said stent isalternately radially compressible to a first diameter and radiallyexpandable to a second diameter, said stent having a plurality oflateral openings; e) mounting said stent over the tubular base graftpositioned upon said mandrel, such that the tubular base graft iscoaxially disposed within the longitudinal bore of said stent inabutting contact therewith; f) extruding a PTFE paste dispersion as afilm, expanding the film, sintering the film between 350° C.-420° C.,and forming expanded, sintered PTFE, tape from the film; g) wrapping theexpanded, sintered PTFE, tape around the exterior of said stent to forman outer tubular layer thereon in abutting contact therewith; and, h) ata first predetermined temperature and a first predetermined time, fusingby heating, said base graft, said stent and said outer tubular layerwhile mounted on and secured to said mandrel, whereby said outer tubularlayer and said base graft become attached to one another through saidlateral openings thereby forming said stented graft.
 2. The method ofclaim 1 further comprising the additional steps of: i) removing thestented, tubular PTFE graft from the mandrel; j) radially contractingthe stented tubular PTFE graft to its radially compact configuration ofsaid first diameter; and, k) subsequently re-expanding the stentedtubular PTFE graft to its radially expanded configuration of said seconddiameter.
 3. The method of claim 2 wherein said re-expanding step iscarried out by advancing the stented graft in said radially compactconfiguration onto a mandrel having an outer diameter substantiallyequal to the inner luminal diameter of the stented graft in saidradially expanded configuration.
 4. The method of claim 1 wherein saidstent is provided with a plastic coating that will adhere to PTFE whenheated, and wherein fusing by heating step further comprises causingsaid outer tubular layer and the tubular base graft to become adherentto said plastic coating.
 5. The method of claim 4 wherein said plasticcoating on said stent is a PTF coating.
 6. The method of claim 5 whereinsaid PTFE coating is formed on said stent by the steps of: immersing thestent in an aqueous PTFE dispersion; removing the stent from the aqueousPTFE dispersion; and, drying the aqueous PTFE dispersion which remainson the stent to form said PTFE coating thereon.
 7. The method of claim 1wherein said wrapping step further comprises helically wrapping PTFEtape in overlapping fashion around the exterior of said stent.
 8. Themethod of claim 7 wherein said PTFE tape has a width of ½ inch, andwherein said tape is helically wrapped such that 6-8 revolutions of tapeare applied per longitudinal inch of the stented graft.
 9. The method ofclaim 7 wherein said helical wrapping of said tape is performed twicefirst in one direction and then in the opposite direction.
 10. Themethod of claim 7 wherein said helical wrapping of said tape results inthe formation of a tubular outer layer which comprises eight layers ofsaid tape.
 11. The method of claim 7 wherein said helical wrapping ofsaid tape is initially performed in a first direction, and subsequentlyperformed in a second direction, opposite said first direction.
 12. Themethod of claim 1 further comprising the step of: depositing polymerparticles between said tubular base graft and said outer tubular layerto facilitate attachment of said tubular base graft to said outertubular layer and to said stent.
 13. The method of claim 12 wherein saidpolymer particles are PTFE.
 14. The method of claim 12 wherein saiddepositing of said polymer particles is carried out by depositing anaqueous polymer suspension between said tubular base graft and saidouter tubular layer.
 15. The method of claim 12 wherein said polymerparticles are deposited by rolling the mandrel having said tubular basegraft and said stent disposed thereon in powdered polymer resin toaccomplish deposition of said polymer particles on the stent and theouter surface of the tubular base graft.
 16. The method of claim 15wherein said powdered polymer resin is PTFE.
 17. The method of claim 1further comprising the step of: affixing the ends of the tubular outerlayer, and tubular base graft to the mandrel to prevent longitudinalshortening during the step of fusing by heating.
 18. A method ofmanufacturing an externally stented, tubular, PTFE graft which isalternately deployable in a radially compact configuration having afirst diameter and a radially expanded configuration having a seconddiameter, said method comprising the steps of: a) extruding a PTFE pasteto form a tubular PTFE extrudate; b) longitudinally expanding thetubular PTFE extrudate to form a tubular base graft; c) completelysintering the tubular base graft; d) positioning the expanded,completely sintered tubular base graft on a generally cylindricalmandrel; e) providing a generally cylindrical stent which is alternatelyradially compressible to a first diameter and radially expandable to asecond diameter, said stent having a plurality of lateral openingstherein; f) positioning the generally cylindrical stent over the tubularbase graft, upon the mandrel, such that the tubular base graft iscoaxially disposed within the stent, and is in abutting contacttherewith; and g) heating the mandrel-borne base graft and stent tocause the tubular base graft to become affixed to the stent, therebyforming an externally stented, tubular PTFE graft.
 19. The method ofclaim 18 further comprising the additional steps of: h) removing theexternally stented, tubular PTFE graft from the mandrel; i) causing theinternally stented tubular PTFE graft to be reduced to its radiallycompact configuration of said first diameter; and, j) subsequently fullyre-expanding the stented tubular PTFE graft to said radially expandedconfiguration of said second diameter.
 20. The method of claim 19wherein the step of fully re-expanding is carried out by advancing theexternally stented PTFE tube graft onto a mandrel which has an outerdiameter of the externally stented PTFE tube graft in its radiallyexpanded configuration of said second diameter.
 21. The method of claim18 wherein the step of providing a generally cylindrical stent furthercomprises coating the stent with a polymer coating which will adhere toPTFE when heated, and wherein the step of heating causes the tubularbase graft to become adherent to the polymer coating formed on thestent.
 22. The method of claim 21 wherein said polymer coating on saidstent is a PTFE coating.
 23. The method of claim 21 wherein said polymercoating is formed on said stent by the steps of: immersing the stent inan aqueous particle dispersion; removing the stent from the aqueousparticle dispersion; and, drying the aqueous polymer dispersion whichremains on the stent to form said polymer coating thereon.
 24. Themethod of claim 23 wherein said aqueous polymer particle dispersion isan aqueous dispersion of PTFE particles.
 25. The method of claim 21wherein said polymer coating is formed on the stent by electron beamdeposition.
 26. The method of claim 18 further comprising the step of:affixing the ends of said base graft and said stent to said mandrel toprevent longitudinal shortening during the step of heating.
 27. Themethod of claim 1 wherein said stent is a self-expanding stent.
 28. Themethod of claim 27, wherein said self-expanding stent comprises a shapememory alloy that can alternately exist in a first and a secondcrystalline state, wherein said stent assumes a radially expandedconfiguration when said shape memory alloy is in said first crystallinestate, and a radially compact configuration when said shape memory alloyis in said second crystalline state.
 29. The method of claim 1 whereinsaid stent is a pressure-expandable stent.
 30. The method of claim 1,wherein said first predetermined temperature is about 350°-370° C. andsaid first predetermined time is up to about 30 minutes.
 31. The methodof claim 30, wherein said first predetermined temperature is about 363°C.
 32. The method of claim 2, wherein said re-expanding step comprisesself-expansion.
 33. The method of claim 6, further comprising heatingsaid stent with said dried PTFE coating thereon at about 350°-370° C.for up to about 10 minutes.
 34. The method of claim 1, wherein the stepof sintering the extruded PTFE paste dispersion film is done at atemperature of about 370° C.
 35. The method of claim 1, wherein the stepof sintering the extruded PTFE paste dispersion film is done at atemperature of about 390° C.
 36. The method of claim 1, wherein the stepof sintering the extruded PTFE paste dispersion film is done at atemperature of about 410° C.
 37. The method of claim 1, furthercomprising the step of: depositing polymer particles between the tubularbase graft and the outer tubular layer which are subsequently meltedduring the step of fusing by heating to promote attachment of thetubular base graft to the outer tubular layer.
 38. The method of claim37 wherein said polymer particles are PTFE.
 39. The method of claim 37wherein the step of depositing comprises applying a liquid polymerparticle dispersion to one of said base graft and said outer tubularlayer, prior to assembly thereof.
 40. The method of claim 37 wherein thestep of depositing comprises depositing polymer particles between saidtubular base graft and said outer tubular layer by applying a liquiddispersion of polymer particles to the exterior of said outer tubularlayer, such that the polymer particles contained within the dispersionwill migrate inwardly through the outer tubular layer.
 41. A method formanufacturing an integrally stented, tubular, PTFE graft that isalternately deployable in a radially compact configuration having afirst diameter and a radially expanded configuration having a seconddiameter, said method comprising the steps of: a) extruding a PTFE pasteto form a tubular PTFE extrudate; b) longitudinally expanding thetubular PTFE extrudate to form a tubular base graft; c) positioning thetubular base graft on a generally cylindrical mandrel; d) providing agenerally cylindrical stent having a longitudinal bore extendingtherethrough, wherein said stent is alternately radially compressible toa first diameter and radially expandable to a second diameter, saidstent having a plurality of lateral openings; e) mounting said stentover the tubular base graft positioned upon said mandrel, such that thetubular base graft is coaxially disposed within the longitudinal bore ofsaid stent in abutting contact therewith; f) helically wrapping theexpanded, sintered PTFE, tape around the exterior of said stent, in afirst direction and then in a second opposite direction, to form anouter tubular layer thereon in abutting contact therewith; and, g) at afirst predetermined temperature and a first predetermined time, fusing,said base graft, said stent and said outer tubular layer while mountedon and secured to said mandrel, whereby said outer tubular layer andsaid base graft become attached to one another through said lateralopenings thereby forming said stented graft.
 42. The method of claim 41wherein said helical wrapping of said tape is performed twice in thefirst direction and then once in the second direction.
 43. The method ofclaim 41 wherein said PTFE tape has a width of ½ inch, and wherein saidtape is helically wrapped such that 6-8 revolutions of tape are appliedper longitudinal inch of the stented graft.
 44. The method of claim 41wherein said helical wrapping of said tape results in the formation ofan outer tubular layer which comprises eight layers of said tape.